The mechanisms by which organisms alter their growth and development in response to changes in their ambient environment are largely unknown. Plants exhibit an enormous array of phenotypic plasticity because most plant organs do not arise until after the seed germinates, allowing organ size and shape to be optimized to the local environment. Because they are sessile and photosynthetic, plants are especially attuned to their light environment. Light influences every developmental transition from seed germination to flowering, having particularly dramatic effects on the morphogenesis of seedlings where it alters the expression of thousands of genes within a few hours. Light signals do not act autonomously, but are integrated with seasonal/diurnal changes in temperature, as well as with intrinsic programs to specify correct spatial and temporal regulation of gene expression, organelle development, and cellular differentiation. The proposed studies aim to understand how one photoreceptor, phytochrome B (PHYB), influences the development of plants that are in competition with other plants for light. Genetic and biochemical approaches have identified a number of proteins that act in close proximity to PHYB under a variety of different growth conditions, yet mechanistic details of how these interactions regulate growth are lacking. In previous years of this grant, it was shown that the synthesis and transport of an endogenous plant hormone, auxin, is altered by light quality cues. The primary goals are to: (1) Determine the mechanism by which auxin homeostasis is altered by shade light; (2) Link phytochrome to auxin-regulated growth promotion by a better understanding of the positive regulator, PIF7; (3) Identify the auxin-independent inputs to shade avoidance. The diverse responses that plants have to light provide a unique model system for understanding phenotypic plasticity. The system combines developmental complexity and sophisticated genetics with the ability to reversibly activate the receptor. As a result, the study of light signaling in plants has led to the discoveryof proteins that regulate DNA damage, transcription, and lipid metabolism in metazoans. Defining common and unique themes in signal transduction between animals and plants will thus explain the origin of key regulatory proteins and may help predict ways in which they can be altered for human benefit.